In the oil and gas industry, optical fibers are increasingly being employed in sensing systems. Based on the topology and configuration of the system, fiber-optic sensors can be single-point, multi-point, or distributed sensors. In a single-point sensor, the sensing element is typically located at the tip of the fiber, but in some cases can be located at any single point in the fiber. In a multi-point sensor, two or more sensing elements are included at specific locations of the fiber. Depending upon the application for which the sensing system is employed, the two or more sensing elements may be physically near or far from one another. In a distributed sensor, the sensing element includes the entire length of the fiber, i.e., the whole fiber acts as a sensor.
One application that uses a distributed sensor is distributed acoustic sensing (DAS), which provides near real-time measurements over the entire length of the optical fiber. As such, DAS may be implemented for pipeline monitoring purposes to measure flow, seismic signals, and leak indicators. Unlike single or multi-point sensors, a distributed sensor provides thousands of sensing points along a single optical fiber.
As a signal traverses optical fiber, the signal suffers from increased attenuation as the length of the fiber increases. Although higher power sources may be used to counteract the attenuation, this approach increases undesirable non-linear effects in the fiber. Additionally, the signal is subject to dispersion, which is a widening of the signal as the length of the fiber increases such that subsequent signals may overlap. The overlapping of signals decreases the information rate of which the fiber is capable. Thus, the dispersion along with nonlinear effects places a maximum limit on the length of the optical fiber that may be used in many applications. Such a maximum limit is particularly detrimental to distributed sensing as thousands of potential sensing points are sacrificed.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.